The invention involves measurement, while drilling, of the earth's magnetic and gravity fields as a reference frame to determine the orientation of a tool in the borehole. Also, the tool senses anomalies in the static magnetic field caused by a ferromagnetic target body or senses a time varying magnetic field created by the injection of an electrical current at some remote point or in the vicinity of a target body. The measurements of the anomalies are used to determine direction and distance, or range, from the position of the tool in the off-vertical borehole to the target. The tool of this invention is also able to provide data that enables simultaneous steering and ferromagnetic detection for controlling the trajectory of the borehole as may be desired, although the steering tools in a wireline configuration will not be used while drilling with a rotating drill string. In this case the drilling rotation would be stopped momentarily while the steering and ranging information or survey was done.
Where the drillstring is not rotating but instead the drill bit is turned by a mud motor at the bottom of the hole, the tool may be used in either a "steering tool" configuration with wireline telemetry to and from the surface, or in the "MWD" measurement while drilling configuration such as that using mud pressure pulse telemetry to and from the surface. All of the steering information would be available without the need for momentarily stopping the drilling to take the ranging survey. The apparatus also includes a sensor or sensors for correlating formation bed interface or measuring geologic strata encountered, apparatus for electric or gamma ray, or radioactive logging, and a telemetry system for transmitting the data to the surface for collection and use, and for transmitting control instructions to sensor or downhole microprocessors from the surface to the tool. The combination of sensors for formation evaluation with the magnetic and gravitational reference frames using magnetometers and accelerometers improves the quality of the information available to the explorers of the reservoir during downhole continuous drilling operations. The magnetometer and accelerometer arrangement can be used both for steering the downhole tool and, in conjunction with the formation evaluation equipment, for directional borehole surveying or for correlation with offset wellbores.
The drilling system involved in drilling an off-vertical well typically includes a turbine driver, or "mud motor", and a rotating bit which is connected to a non-rotating drillstring by means of a short, slightly bent section of drill pipe, or "bent sub", an articulated assembly, or a jetting assembly. Alternatively the housing of the turbine may be bent so as to cause the rotating axis of the bit to be slightly out of alignment with the borehole. The orientation of this system may be altered under surface control to advance the borehole along an accurately defined course for interception or avoidance of a target. When drilling such an off-vertical well, it is often necessary to change its course, or path, by controlling the position of the drill itself. The directional drilling processes require a precise knowledge of the orientation of the drill and its path so that the course may be evaluated and decisions made concerning alterations of the borehole path.
A device, known in the art as a steering tool, provides such measurements of the orientation of the drill in the borehole with respect to a reference system while the drilling operation is underway. The steering device usually includes component or directional type magnetometers and accelerometers for sensing the earth's magnetic and gravity fields, thus to provide the required reference framework. The steering device is secured with respect to the drill with a key or other means and thus fixes the magnetic and gravity reference frame to the drill itself.
The device is mounted in a non-magnetic region sufficiently removed from the drill and other ferromagnetic components in order to respond only to the magnetic fields external to the drill system. As the drilling proceeds, the device measures the components of the magnetic and gravity fields and telemeters the measurement signals to the surface. At the surface they are resolved by suitable computation to provide the driller with values of the orientation of the drill system and/or trajectory of the wellbore.
A representative steering tool is described in U.S. Pat. No. 3,791,043. This steering tool consists of a triaxial component magnetometer to measure the earth's magnetic field and two bubble inclinometers to measure the gravity field. Surface instrumentation resolves the magnetic and gravity measurements to provide readings of the azimuth and inclination of the drill to the driller as well as orientation of the bent sub with respect to the high side (or vertically topmost point) of the hole (toolface). The drill orientation is changed as appropriate in accordance with the readings to adjust the course of the borehole. This U.S. Pat. No. 3,791,043 is specifically incorporated herein by reference.
It is sometimes necessary to determine the direction and range to a nearby subterranean ferromagnetic body from a position in the borehole being drilled. For example, adjacent preexisting wells must be sensed and avoided when a borehole is being drilled as when multiple wells are being drilled from a common small spaced platform. The accuracy of the sensors of the magnetic ranging system, in combination with a telemetry system that makes readings available to the surface operators while drilling is in progress, is especially suited for avoiding existing wells. Yet another example would be the search for subterranean ore bodies exhibiting magnetism, or those which do not exhibit ferromagnetism. This combination tool will be useful for drilling a pilot shaft to search for certain conductance properties in applications such as gold or silver mining. Another application could reveal large coal streaks by the nonconductance or resistance encountered. Another example is when the borehole being drilled is an off-vertical relief well being drilled to intercept a blowout well at a depth below the disturbance caused by the blowout. U.S. Pat. No. 4,072,200 is directed to this application and includes magnetometers which detect remnant or induced static magnetic field anomalies resident in the ferromagnetic body involved in the second well or body to be located, or alternately magnetometers which detect a time varying magnetic field created in the well or target body by an electrical current injecting system controlled by the operator. As appropriate, parts of the disclosure of U.S. Pat. No. 4,072,200 are included as part of the disclosure of this application. All of U.S. Pat. No. 4,072,200 is specifically incorporated in this application by reference.
The techniques of drilling one well to intercept another with a target well exhibiting a signal is taught in U.S. Pat. Nos. 3,285,350 and 3,731,752.
Given the case where the target is inaccessible, as in a blowout situation, a method for an approach to the target location is disclosed in U.S. Pat. No. 3,725,777, in which the total magnetic field and magnetic compass readings were made and compared through least squares fit analysis to various assumed positions and magnetizations of the target well casing. However, this technique is subject to location ambiguities.
The measurement of magnetic vector components, as taught in U.S. Pat. No. 4,072,200, allows the determination of target polarity as well as direction and resolves such location ambiguities. In addition, the method of determining range from the tool to the target, using gradient measurements, is applicable for both the static and time varying field techniques.
As shown in the prior art, there have been provided separate tools for steering drilling, and for borehole orientation; anomaly seeking magnetic tools; for either residual and/or time varying magnetic field tools, and formation evaluation tools. Each such tool requires a separate survey to be run with the resulting time and complexity involved. Each operation involves stopping the drilling operation, removing the drillstring with the steering tool or MWD, running a survey with the ferromagnetic body seeking system, returning the drillstring to the well bore, and then continuing the drilling operation. The procedure is expensive, time consuming, complex, and subject to introduction of error. In addition, the bore hole erodes with time.
The combination of the steering tool with the ferromagnetic body seeking system, or magnetic ranging components, into a single tool that operates while drilling and which may include a range of formation evaluation or correlation sensors is a significant advance over the equipment presently available in the industry. Some advantages of the combination are the enhanced orientation capabilities of the ferromagnetic sensing systems, the cost and time savings of the continuous drilling and surveying compared to trips out of the hole for running separate surveys, and the improved quality of the formation evaluation which results from the more pristine borehole condition as measured immediately after drilling in contrast to measurements taken after a drillstring is tripped out when caving, sloughing, and invasion may distort the evaluations. Drilling fluids can be highly caustic, and tend to react with shales, leading to sloughing of formation materials and tool sticking.
Additional advances in safety over the prior tools are especially needed, and are provided by the present invention, in the circumstances of drilling to intercept a blowout well. Any open hole survey operation entails a certain degree of risk, however, hydrostatic conditions encountered in relief well drilling are often abnormal and irregular, and consequently the potential for blowout is higher. The combination tool in either an electric line or a MWD configuration eliminates the need to come out of the borehole with the drillstring so that survey data may be obtained, thus reducing the likelihood of another blowout. Since the drillstring remains in-hole and fluid circulation continues, it is easier to balance or control the hydrostatic pressures, eliminating a common cause of blowouts.
Further, advantages to the combination tool are capabilities not found presently in the industry. The magnetic ranging orientation system has a resolution allowing directional drilling accuracy. All directional drilling operations require some method of orienting the downhole tool and the high quality, magnetic sensors in the combination tool provide accurate information for orienting the tool. In combination with a directional drilling assembly the magnetic ranging system will operate in a borehole through unusually high inclinations through which it would be impossible to run a wireline survey. At borehole inclinations near the limit of operability for wireline survey, formation evaluation information in real time is very valuable, because it provides a record of information about the formation even if the well blows out, or the tool is stuck, or the hole is lost for any other reason. Even when the drilling assembly is lost, the components of this composite survey system can be retrieved for reuse, or at least the survey data can be retrieved or retained.
Further important improvements over present practice are the pinpointing of responsibility for the drilling and survey operations and a reduction in personnel and therefore a reduction in costs resulting from the combination of steering, magnetic ranging and formation evaluation into a single system operated by a single crew. A very significant advantage also results from transmitting the magnetic ranging information while drilling rather than obtaining it in intervals as is furnished by present wireline tools. Drilling must be stopped to run the wireline tools, so the tendency is to overdrill before stopping to evaluate the formation and determine the trajectory of the borehole. Because the drilling has stopped and the entire rig is idle during the wireline survey and following period when the data is analyzed, there is pressure on the person or persons doing calculations for determining target position based upon large quantities of data taken over long intervals of drilling and corresponding to a substantial length of the borehole. Consequently, mistakes in calculations are more likely. With continuous data transmission occurring during drilling operations, the progress and trajectory can be monitored and the calculations made much more efficiently without the mistake causing pressure posed by an expensive idle drilling rig.
It is highly desirable and advantageous to be able to combine the operations which provide steering information with measurements concerning the range and direction to a subterranean body as well as wellbore trajectories and formation evaluation. The present invention provides this desirable combination by including the static magnetic, gravity, and time varying magnetic measurements in a drilling system. The invention also provides the capability for accomplishing other well and formation measurements including electric or gamma logging, formation temperature, etc., during the drilling operations. This feature provides drilling personnel with the capability for immediately and continuously receiving well and formation data to ensure the safety and efficiency of the drilling operation. This information is available to be used in combination with formation evaluation devices which require knowledge of the earth's magnetic and/or gravity fields for orientation. The information measured is converted to a single serial composite signal and transmitted from the subsurface tool to the earth's surface by suitable telemetry means such as by hydraulic signal pulses in the drilling fluid column, by a single electrical conductor, solid state memory with periodic readout, or the like. With the tool fixed into the drilling system, the invention also provides the necessary information to serve the drilling steering purpose.
In the past, acquisition of well survey signals at the surface was dependent on the number of signal electrical conductors extending from the downhole sensing tools in the drilling zone, to the surface. For example, an electrical signal transmission cable having seven conductors has been widely used, thus limiting the acquisition to seven signals. The present invention incorporates signal processing circuitry in the downhole tool which receives and measures any number of selected signals relating to conditions of the well bore and formation. These signals are received as analog signals and the circuitry of the down hole tool digitizes and multiplexes them and processes the signals by way of a microprocessor to form a single composite signal. This single signal can then be transferred to surface signal processing and display equipment by a variety of signal transmission systems such as single electrical conductor, by mud pulse fluid signal transmission, by retention in solid state memory for later retrieval, or by any other suitable means.